Some cysteine proteases such as papain and those of mites and schistosomes have potent allergenic properties. To clarify the allergenicity of nematode cysteine proteases, the enzyme was purified from the intestinal nematode Nippostrongylus brasiliensis using cation exchange chromatography and gel filtration chromatography. The purified protease, of 16 kD and pI 8.5, showed maximum enzyme activity at pH 5.5 and substrate preference for Z-Phe-Arg-MCA. The specific inhibitors of cysteine protease leupeptin, iodoacetic acid, and E-64, completely suppressed the activity, indicating that the purified enzyme belongs to the cysteine protease family. Cysteine protease activity was found not only in somatic extract, but also in the excretory-secretory (ES) product of the nematode. When anti-cysteine protease immunoglobulin isotypes were examined in sera from rats infected with N. brasiliensis, a high level of IgG1 and a lower level of IgE antibody were detected. Depletion of IgG antibodies from the sera using protein G affinity columns resulted in a marked increase in reactivity of anti-cysteine protease IgE with the antigen, possibly due to the removal of competing IgG antibodies. In contrast to IgE and IgG1, production of anti-cysteine protease IgG2a was negligible. These results indicate that the nematode cysteine protease preferentially evokes an IgE/IgG1 antibody response.